This invention is directed to a technique for monitoring the eye movements of a subject viewing a certain scene in order to determine at what portion of the scene the subject is looking and, more particularly, the invention enables the signals obtained by the eye-movement monitoring equipment to be corrected for movement of the subject's head.
Eye-movement monitoring equipment is in use for a variety of purposes. In the medical field, it is a valuable tool for the diagnosis of visual and mental impairment. In the field of advertising, subjects can be tested for their reactions (e.g. interest, readership, reexamination, recall) to various forms of advertisements such as magazine ads, billboards, television ads and point-of-purchase displays. Other actual and potential uses exist in sports, the military, and school reading classes.
The measurement of eye movement can be done based on any one of a number of principles. These are dicussed in the article "Methods and Designs, Survey of Eye Movement Recording Methods" by Lawrence R. Young and David Sheena in Behavior Research Methods & Instrumentation 1975Vol. 7(5) pgs 397-429. The principle utilized in the present invention is "corneal reflection". Briefly, incident light is reflected by the convex surface of the corneal bulge in a pattern of diverging light. This is imaged through a concave lens into a sensor such as a video camera, and the resulting signal is used to obtain a position measurement.
Eye-movement monitoring apparatus using the corneal reflection principle can be divided into the following two categories: a stationary type and a head-mounted type. In the former, a stationary light source directs a light beam at the eye. The beam reflected by the cornea is detected by a sensor, also stationary. See pages 402-403 of the above-mentioned article by Young and Sheena, the article "Eye Fixations Recorded on Changing Visual Scenes by the Television Eye Marker" by J. F. Mackworth and N. W. Mackworth in the Journal of the Optical Society of America Vol. 48, No. 7, July, 1958, pgs 439-445, and U.S. Pat. No. 4,075,657. In the second category of eye-movement monitoring equipment, both the light source and sensor are mounted on the subject's head. Such equipment has various advantages over the stationary variety in terms of, for example, size, weight, versatility, cost, and portability. Nevertheless, it works basically the same way in that a beam of light from a source is reflected off the cornea and into a sensor. Examples are found in pages 404-409 of the Young and Sheena article as well as U.S. Pat. Nos. 4,102,564 and 3,542,457.
The signals from the eye-movement monitoring sensor must, of course, be calibrated and suitably processed to correlate the subject's eye movements with a particular portion of the scene viewed. This is done by well known circuitry. In systems in which the eye movements and scene are shown simultaneously on a video monitor, the positioning circuitry responsive to the eye movement signals superimposes a cursor over the scene.
One disadvantage of eye-movement monitoring techniques is the requirement to eliminate, or certainly sharply minimize, the head movements of the subject. Such head movement produces an erroneous interpretation of what portion of the scene is being viewed by the subject. The impact of head movement on the accuracy of measuring eye movement with stationary type equipment is well known. A mathematical analysis can be found on pages 402-403 of the Young and Sheena article. U.S. Pat. No. 3,712,716 mentions that a lateral head movement of 0.2 mm while viewing an object two meters away changes the position being looked at, so far as the measuring apparatus is concerned, by 35 mm. A similar analysis can, of course, be readily made for the head-mounted type of eye movement monitoring equipment.
In order to prevent such erroneous readings, various devices have been used to restrict head movement. This includes chin holders, bit boards and head straps. In addition, some movement detection and correction techniques are also known as disclosed on pages 420-423 of the Young and Sheena article. However, the head-movement restriction devices are uncomfortable, particularly if the subject is confined for any significant period of time. Such discomfort can cause distraction and this also detracts from the results obtained with the eye-movement monitoring equipment.
The known signal correction techniques are also unsatisfactory because the required devices and circuitry are too complex and costly. In addition, the overall arrangement can be bulky and, therefore, not well suited to work in settings requiring portability.